Combined DFT and microkinetic modeling study of SO₂ hydrodesulfurization reaction on Ni₅P₄ catalyst

Optimizing catalysts for the SO₂ hydrodesulfurization (HDS) is a crucial step toward conforming with the environmental requirements concerning SO₂ emissions. Nickel phosphides have been reported as efficient catalysts in HDS reaction. However, how HDS reaction proceeds on nickel phosphides is not well understood. On this context, the present work focuses on the mechanistic understanding of SO₂ HDS reaction over Ni₅P₄surfaces. The adsorption of both reactants, SO₂ and H₂ molecules, on low-index facets of Ni₅P₄ crystal, namely (0 0 1), (0 1 1), (1 1 1), (1 1 0), (1 0 1), (0 1 0) and (1 0 0) surfaces, were investigated using density functional theory (DFT) calculations. The stability of Ni₅P₄ surfaces was examined and (0 0 1) surface was found to be the most stable surface. Therefore, microkinetic modeling was conducted on Ni₅P₄(0 0 1) surface to predict the catalytic preferred pathways. Reaction towards H₂S, main product, exhibited 100% selectivity at reaction temperatures below 700 K, however the selectivity towards H₂O became dominant at higher temperatures. This is because the barrier for HS^- hydrogenation to H₂S (1.20 eV) is lower than that of the OH hydrogenation to H₂O (2.37 eV). The model revealed that conversion of HS^- ions to H₂S was the rate-controlling step at reaction temperature below 500 K, whereas H₂S desorption dominates the overall reaction rate at higher temperatures. The apparent activation energy of HDS reaction decreased considerably from 195 to 48 kJ/mol at reaction temperature range of 400–800 K. The reaction orders in SO₂ and H₂ increased with rising temperature, reaching 0.15 and 1.0, respectively, at 800 K.